1. Field of the Invention
The present invention relates to chromatography and more particularly to a chromatographic separation material, method to make the chromatographic separation material and a method to use the chromatographic separation material. The chromatographic separation material is especially suited for chiral enantiomeric separations.
2. Prior Art
In the past few decades, chiral enantiomeric separations have gained great attention among many chromatography separations. While over a hundred types of chiral separation materials have been developed, only a few types/classes dominate the field of chiral separations in industry. These materials are made by coating or covalently bonding chiral selectors to supports, typically inorganic silica gel supports. Examples of the dominant classes of chiral selectors include polysaccharide, macrocyclic antibiotic, and π complex.
Cyclofructans (CFs) are chiral selectors by bonding it to silica gel in US2011/0024292. They are a group of macrocyclic oligosaccharides consisting of six or more β-(2→1) linked D-fructofuranose units. A common shorthand nomenclature for these compounds is CF6, CF7, CF8, etc., where the number denotes the number of fructose moieties (i.e., 6, 7, 8, etc.) in the cyclic oligomer.
Cyclofructans were first reported by Kawamura and Uchiyama (Kawamura, M. and Uchiyama, K. Carbohydr. Res., 1989, 192). As the reference describes, cyclofructans can be produced via fermentation of inulin by at least two different strains of Bacillus circulans. The gene that produces the cycloinulo-oligosaccharide fructanotransferase enzyme (CF Tase) has been isolated, and its sequence determined and incorporated into the common yeast, Saccharomyces cerevisiae. Hence, the facile production of CFs is possible. The basic structure of CF6 is depicted in FIG. 1. The x-ray crystal structure of CF6 has shown that the smaller CFs has no hydrophobic cavities as do cyclodextrins. Consequently, hydrophobic inclusion complexation, which plays an important role in the association of organic molecules with cyclodextrins, does not seem to be relevant for cyclofructans.
The pentose moieties (fructoses) of CFs alternatively form a propeller-like circumference around a crown ether core unit. For example, the crystal structure of CF6 reveals that six fructofuranose rings are arranged in a spiral or propeller fashion around the 18-crown-6 core, oriented either up or down toward the mean plane of the crown ether. Six three-position hydroxy groups alternate to point toward or away from the molecular center, and the three oxygen atoms pointing inside are very close to each other (˜3 Å). It is clear that there is considerable internal hydrogen bonded hydroxy groups. The other side of CF6 appears to be more hydrophobic, resulting from the methylene groups of —O—C—CH2—O— around the central indentation. A computational lipophilicity pattern of CF6 also confirms clear “front/back” regionalization of hydrophilic and hydrophobic surfaces. Both the crystal structure and computational modeling studies demonstrate that CF6 appears to have considerable additional internal hydrogen bonding interactions. Therefore, CF6 is different from other 18-crown-6 based chiral selector for reasons of (i) three 3-OH groups completely cover one side of the 18-crown-6 ring and (ii) the core crown oxygen are almost folded inside the molecule.
US2011/0024292 (WO2010/148191) discloses examples of separation performance of CFs bonded to solid silica gel as a separation material. While the '292 application teaches CFs as a chiral selector, the supporting material, namely silica gel, has drawbacks. Silica gel may not be best suited for chiral and hydrophilic interaction liquid chromatography (HILIC) separations. In addition, silica gel has a limitation in durability against using alkali solvents, in cost for bulk material and ease of scale-up.